1. Field of the Invention
The present invention relates to a stereo multiplex demodulator which is suitable for application to an FM stereo receiver and the like.
2. Description of the Prior Art
In present FM stereo broadcasting, a main carrier signal is frequency-modulated by a composite stereo signal Sc which is expressed as
Sc=Sm+Ss+Sp PA1 Sm=L+R PA1 Ss=(L-R) sin .omega.t PA1 Sp=k sin .omega.t/2 . . . pilot signal PA1 L : left=channel audio signal PA1 R : right-channel audio signal PA1 .omega. : 2.pi.ft PA1 f : 38 kHz . . . subcarrier frequency PA1 k : constant
Therefore, in the FM receiver, a switching signal synchronized with the pilot signal Sp is generated with the frequency f, and this switching signal is used to switch the composite signal Sc, thereby demodulating the signals L and R.
When the stereo signals are demodulated by switching as described above, the switching signal is a rectangular wave signal, and contains a higher harmonic component of an odd-numbered order. Further, the composite signal Sc contains a higher harmonic component due to the non-linearity of the signal system, and it is frequently observed that the composite signal Sc contains a beat frequency component for the adjacent channel and an intermediate frequency signal component.
Thus, when the stereo signals are demodulated, an undesired signal component contained in the composite signal Sc is switched by the higher harmonic component of the switching signal so that the stereo demodulated output contains a beat interference component caused by the undesired signal component in addition to the original signals L and R.
In some FM receivers, the composite signal Sc is supplied to a low-pass filter to thereby remove the undesired signal component contained in the composite signal Sc. However, such low-pass filter needs to have a steep cut-off characteristic, which results in increased cost. Further, when the composite signal Sc is processed by the low-pass filter, the phase and the frequency characteristic of the composite signal are affected by the low-pass filter, with the result that distortion is increased and the separation is made less effective.
It has also been proposed that the composite signal Sc be switched only by the higher harmonic component of the switching signal, and that the beat interference component contained in the original stereo demodulated output be canceled by the switched output. However, for this proposal to achieve its desired results, the level of the beat interference component used to cancel the beat interference component must accurately coincide with that of the beat interference component contained in the stereo demodulated output. Therefore, strict or very accurate adjustment is needed.